1. Field of the Invention
This invention relates to a method used in a digital scanner, and in particular, it relates to a method and apparatus for setting optimum noise filter for a scanner used in a closed loop system.
2. Description of the Related Art
Barcodes are a form of machine-readable symbology for recording digital information. One-dimensional and two-dimensional black and white barcodes have been widely used. Color barcodes have also been proposed. One-dimensional barcodes typically contain a series of parallel lines of varying widths. Two-dimensional barcodes may contain stacked rows of parallel lines of varying widths, or a two-dimensional arrangement of tiles. To increase data capacity and reduce the physical size of barcodes, it is desirable to decrease the width of the lines or the size of the tiles in a barcode. Reduction in width or size of the barcode lines or tiles, however, is limited by various hardware and software factors. One limiting factor is the nominal spatial resolution of the printer that prints the barcode and the scanner that scans the barcode. Another type of limiting factors are the inherent noises of the printer and the scanner, including toner scattering in the printer, settings of the imaging algorithm in the scanner, etc. Toner scattering is a sort of unintended toner adhesion in the vicinity of printed features or a background area that should be white, and may cause blurring of fine lines, fine points and sharp edges. Digital scanners typically are equipped with software noise filters to digitally filter out certain features in the scanned data that are considered noise. For example, a noise filter may remove isolated black pixels in an otherwise white area as noise. Such noise filters may use various models to model noises in printers and attempt to remove the noises based on the noise model.
Another source of noise or artifacts in scanning a barcode with fine lines or small tile sizes is skewing in the printer. When the size of the barcode elements (lines or tiles) are as small as the size of a few printed dots, skewing may have a large effect on the detected shape or size of the barcode elements.